The present invention relates to optical modules for optical communications, and more particularly, to optical modules suitable for 25-Ω driving.
The optical communications modules using a semiconductor laser are one of the key devices in transceivers for optical-fiber transmission. Along with the proliferation of broadband networks in recent years, optical communications modules have been speeded up and types up to 10 Gbits/s in bit rate are coming to be most commonly used. Optical communications modules suitable for the above applications are required to be more compact and less expensive as well as to achieve higher transmission waveform quality.
Japanese Patent Laid-Open No. 2001-308130 describes a module that simultaneously ensures the retention of a 3-dB band in the small-signal passage characteristics (S21) of the optical modulator of a modulator-integrated light source unit and reduction in the small-signal reflection characteristics (S11) of the modulator. The above module is realized by properly conditioning the relationship in inductance between a first bonding wire for connecting the modulator and signal line of the modulator-integrated light source unit, and a second boding wire for connecting the modulator and a matching resistor.
Also, the “ASIP 1310 nm EML TOSA”, a pamphlet of ASIP Inc., lists electroabsorption laser modules mounted in the CAN-type packages having a driving impedance of 50 Ω (ohms) and a termination resistance of 100 Ω.
Packages of the CAN type are in need for further reduction in the dimensions of optical communications modules.
When the CAN type of package is used, the output impedance of a driving IC and the impedance of the transmission line between the driving IC and the package must be adjusted to a 20 Ω–30 Ω range to obtain better transmission waveforms. The reason is described below. The CAN-type package has its lead pins hermetically sealed with glass and fixed, and a coaxial line is formed between the sealed sections. The characteristic impedances of these sealed sections usually range from 20 Ω to 30 Ω. Waveform deterioration due to multipath reflection can be avoided by matching the impedance of a driving signal route accurately with the impedance of each hermetically sealed section.
To perform modifications for a driving impedance of 25 Ω in Japanese Patent Laid-Open No. 2001-308130, uniform waveform quality can, in principle, be achieved by scaling, i.e., by halving the resistance value of the matching resistor 4, halving the inductances of the first and second bonding wires 5 and 6, and doubling the capacity value of the optical modulator 10. Almost all optical modules currently in use, however, are designed with a driving impedance of 50 Ω. If the original capacity of the optical modulator element in the modulator-integrated semiconductor laser chip of an optical module designed with a driving impedance of 50 Ω is doubled, waveform quality deterioration due to a decrease in frequency band will result since a sufficient band will not be obtainable.
Additionally, according to studies by the inventors, changing the driving impedance to 25 Ω, the termination resistance value to 25 Ω, and the inductances of both bonding wires 5 and 6 to ½, by using the conventional modulator-integrated semiconductor laser chip (suitable for 50-Ω driving) that does not increase the capacity value of its optical modulator, causes great peaking of the frequency characteristics in small-signal passage characteristics S21, thus deteriorating transmission waveform quality. As described above, with existing technologies, high waveform quality has not been achievable at a driving impedance of 25 Ω by using a modulator-integrated semiconductor laser chip designed with a driving impedance of 50 Ω.
This means that some other different type of modulator-integrated semiconductor laser must be designed and manufactured for use at a driving impedance of 25 Ω. In view of design and development expenses, production management expenses, and the like, however, this means increases in the costs of modulator-integrated semiconductor laser chips. The development of such a different type of modulator-integrated semiconductor laser needs to be avoided to obtain more compact, less expensive CAN-type optical transmission modules.